The present invention relates generally to aromatic polyimide thermoplastics protected against discoloration during processing, and to a method for suppressing discoloration of aromatic polyimide compositions.
Discoloration is generally viewed as a detriment to a plastic, since it spoils its appearance, creates the impression of nonuniformity, and is often associated by the customer with inferior properties, whether or not it is associated with actual degradation of the polymer. For some applications such as optical elements, coatings for solar cells, and the like, discoloration seriously impedes or prevents the proper functioning of a device made from a polyimide plastic. For other applications, such as for construction of aircraft interiors, discoloration is primarily an aesthetic detriment.
The art of compounding of plastics has led to a variety of stabilizers and discoloration preventatives, which tend to differ from plastic to plastic. In the foregoing and following discussion, the terms "preventing", "prevention" and "preventative" are used in a relative sense rather than in an absolute sense, i.e. by prevention is meant suppression relative to an untreated control sample.
Vinyl polymers such as PVC are successfully prevented from yellowing by alkyltin compounds, barium-cadmium salts, and calcium-zinc salt mixtures. These stabilizers are believed to retard formation of conjugated double bonds which cause color. In polyolefins (which can degrade without a great deal of discoloration), the required stabilization is more directed to preventing autooxidation with loss of mechanical properties than to preventing discoloration. Various phenols in combination with phosphites or organic sulfides are commonly used for this purpose. In styrenic polymers such as ABS, it is also common practice to use tertiarybutylphenols plus phosphite. Here again, much of the concern is protection against loss of mechanical properties, although prevention of discoloration is sometimes a consideration. In nylon 6,6, especially for fiber use, discoloration during thermal processing is a serious problem and discoloration preventatives such as phenylphosphinic acid are used. In polyester fiber and film, various phosphorus additives are used to prevent discoloration.
The development of newer thermoplastics processed at high temperatures (such as polyimides) has imposed more stringent needs for improved means for preventing degradation, including that part of degradation which manifests itself as discoloration. The structures of these high temperature thermoplastics and their modes of degradation are different from the earlier plastics mentioned above. Most of these high-performance thermoplastics have heteroatoms (such as nitrogen in the case of polyimides) in the backbone of the polymer, which makes their chemistry quite different from the chemistry of purely carbon-chain polymers. Consequently, it has not been possible to predict whether stabilizers useful in polymers mixed, molded or extruded at lower temperatures, such as below 500.degree. F., would be useful in polymers typically processed at higher temperatures.
This requirement for discoloration preventatives in the processing (hot mixing, molding, and extrusion) of thermoplastics is also quite different from the requirement for stabilizers intended to protect the plastic during its service life, which for obvious reasons must be at temperatures lower than its processing temperature. Those stabilizers which are known for protection against atmospheric oxidation and photodegradation at the service temperatures of the plastic are generally found to be disappointing as high-temperature thermal processing stabilizers, and particularly disappointing in regard to prevention of discoloration during thermal processing.
The polyimide thermoplastics constitute an important class of thermoplastics which are processed at high temperatures. For example, polyimide molding resins are typically molded at 680.degree. to 800.degree. F. (340.degree. to 425.degree. C.). Polyimides which are applied as coatings in the form of polyamic acids are typically heated to about 300.degree. C. to convert the amic acid structures to imide structures. In either case, some coloration usually develops during this thermal treatment.
Various efforts have been made to stabilize polyimides against deterioration of mechanical properties caused by heat. For instance Avakian, U.S. Pat. No. 4,508,861 (Apr. 2, 1985) teaches the use of hindered phenols, aryl phosphonites, organic phosphites, thioesters, and mixture thereof, to stabilize the mechanical properties of polyetherimides, a subclass of polyimides. However, this teaching is silent regarding the color effects of such stabilizers. Avakian does refer to prior art stabilizers for other polymers as being incompatible with polyimides and causing discoloration (see Col. 1, Lines 52-55).
It has now unexpectedly been found that compounds of the alpha-hydroxyketone class, which includes the benzoins, are effective discoloration preventatives for polyimide thermoplastics. This finding is quite surprising since these alpha-hydroxyketones are themselves not exceptionally stable to heat. For example, benzoin was reported by Lachman, J. Am. Chem. Soc. 46, 717-718 (1924) to decompose at 300.degree. C. (572.degree. F.) to benzaldehyde and other products.
It was disclosed in European Patent Application No. 0 176 811 (date laid open: 4.9.86) that certain benzoins are stabilizers for ABS, an acrylonitrile/styrene/butadiene graft copolymer. However, this European application indicates that the anti-discoloration action diminishes rapidly as the processing temperature is raised and becomes quite poor around 300.degree. C. (572.degree. F.). (See FIG. 1 in the cited application).